Method of loading in a multilayer vacuum drier



Patented Aug. 2, 1949 rss raTENT OFFICE ME'EHUD 6F LOADING EN A MULTILAYER VACUUM DRIER No lll rawing. Application May 31, 1944,

Serial No. 538,198

This invention relates to a method of loadin in a multi-layer vacuum dryer, and more particularly to a drying system of the type disclosed in Heineman application, Serial No. 520,773, filed February 2, 1944, now Patent No. 2,441,021, and Baer application, Serial No. 524,177, filed February 28, 1944, now Patent No. 2,457,116.

In apparatus of this type, material to be dried is introduced into an elongated vacuum chamber and is progressively advanced therethro-ugh. During the vacuum treatment the material is carried on trays having a screen bottom roughly 1 root in width and feet long. These trays are advanced horizontally transversely of the long axis of the tray and six layers of trays are superposed. In the preferred form the vertical distance from the center of one tray to the center of the next higher tray is about 8 inches and each tray is surrounded by heating coils which are, on the average, 3 inches from the surface of the material being dried. The material being dried is thus subjected to intense radiant heat, the coils being usually maintained under pressures from 110 lbs. per square inch or less of steam down to about 2 lbs. per square inch or more.

In operation it has been found impracticable to attempt to vary the steam pressures vertically of any particular bank of coils, the variations which are made being longitudinal. That is, the highest temperature coil is at the beginning of the vacuum dryer and the lowest at the end thereof.

The chamber is maintained under a low pressure, preferably less than 100 mm. of mercury absolute. An operating pressure of 57 mm. has been found quite satisfactory. The atmosphere surrounding the product is made up of superheated steam, part of which of course is generated by the product. It is also preferred to introduce additional superheated steam into the drying chamber near each end thereof. Superheated steam is also withdrawn from the central part of the structure.

In operating such a drying system it has been found that the temperature at the top of the dryer tends to rise considerably as compared with the lower portions thereof, and mechanical attempts to adjust this temperature have not been satisfactory. As a result the material on the top trays has a tendency to overdry with attendant scorching or case hardening. This is particularly true of vegetables such as potatoes. On the other hand, if the conditions in the dryer are adjusted 6 filairns. (Cl. 34'15) so that the top layer is properly dried, then material on the lower layers is. unduly moist.

I have found that this difilculty may be overcome very simply in practice by adjusting the loading of the trays in their superposed layers, While theoretically such adjustment, if made, should be applied in all of the drying layers, it has been found that for practical purposes only the top layer need be adjusted. The extent of this adjustment is substantially independent of the tray loadingsthat is, the upper tray is loaded by adding approximately 0.2 lb. of finely divided material to be dried per square foot of screen area. This added amount is independent of the total amount of material per square foot of area in the tray. Thus, if the normal loading in the dryer is 0.8 lb. per square foot of tray, the top layer will be provided with 1 lb. per square foot. On the other hand, if the tray loading on the lower trays is 1.6 lbs. per square foot, the upper tray will be provided with 0.2 lb. per square foot additional, or 1.8 lbs. per square foot.

As an example of the process, trays in all levels were loaded with 1.4 lbs. per square foot except in the top layer which was loaded with 1.6 lbs. per square foot. It was found that the moisture contents on the first, third and sixth layers were approximately equal, after drying, whereas, when the same material was loaded at 1.4 lbs. per square foot throughout, the moisture content on the third and sixth tiers averaged approximately twice that of the material in the upper layer.

In general the addition of 0.2 lb. per square foot to the upper layer does not quite bring the moisture content after drying in the upper level up to that of the lower trays, but it does prevent scorching or hardening and attempts to add considerably more material to the top level, resulting in interference with the uniformity of drying at that level. The material undergoing drying in the aforementioned examples was potatoes which were diced into approximately inch cubes.

The differential of 0.2 lb. is independent of the material being treated, provided the moisture content of the materials at the beginning of drying is approximately the same. For most vegetable and meat products, this is sufficiently true that no adjustment of the amount is required. The amount of adjustment is, however, to some extent the function of the heat capacity of the apparatus as well as the depth thereof. Furthermore in vacuum chambers which have considerably more than six superposed tiers, it may be necessary to adjust more than the top layer.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

What I claim as new and desire to secure by Letters Patent is:

1. In the operation of a vacuum drying system in which the products to be dried are passed progressively through an elongated drying chamber in a plurality of superposed thin drying layers while exposed to an atmosphere of superheated steam under low atmospheric pressure and while subjected to intense radiant heat, the steps of adding suflicient additional material to be dried to at least the upper drying layer as compared with lower drying layers whereby substantial over-drying and deterioration of the product is avoided, introducing superheated steam to said chamber from an external source and withdrawing steam from said chamber.

2. The method as set forth in claim 1, in which the product is a vegetable.

3. The method as set forth in claim 1, in which the amount of excess material added is approximately 0.2 lb. per square foot of drying area.

4. The method as set forth in claim 1, in which the product is a vegetable, and in which the amount of excess material added is approximately 0.2 lb. per square foot of drying area.

5. The method as set forth in claim 1, in which the number of layers is approximately 6, the distance between layers is approximately 8 inches, and the top layer is provided with an additional approximate 0.2 lb. per square foot of drying area.

6. The method of avoiding case hardening or scorching of food products exposed to intense radiant heat in an atmosphere of superheated steam in a vacuum drying system in which the food products to be dried are passed in the same direction through an elongated drying chamber in a plurality of superposed thin drying layers while maintaining an atmosphere of superheated steam in said chamber and within which all of said products are subjected to intense radiant heat, said superheated steam being in part generated during the drying and in large part added from an outside source so as to maintain a current of steam therethrough and the system being under low absolute pressure, the step of adding additional material to be dried to at least the upper drying layer as compared with lower drying layers whereby overdrying case hardening and scorching of the product in the upper layers is avoided.

ROBERT M. SCHAFFNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 888,257 Passberg May 19, 1908 1,284,305 Gammel Nov. 12, 1918 1,410,719 Rea Mar. 28, 1922 1,536,291 Glessner May 5, 1925 1,551,818 Garnsey Sept. 1, 1925 1,577,709 Glessner Mar. 23, 1926 1,668,591 Hoyt May 8, 1928 2,132,897 Gentele Oct. 11, 1938 2,156,845 Gentele May 2, 1939 

